1. Field of the Invention
The present invention relates generally to disk drives, and more specifically to a self-servo writing process.
2. Description of Related Art
Storage devices like hard disk drives (HDD) are widely used in electronic devices, such as computers, MP3 players, video recorders, digital cameras and set-top boxes which need to store a large amount of data. FIG. 1A illustrates a typical HDD 100, including at least one disk 110, which a spindle motor can cause to rotate around the axle of a central drive hub. A read/write transducer or head 130 is attached to a load beam 140 of a suspension via a slider and a gimbal. The load beam 140 is supported by an actuator arm 150 of an actuator 160. In operation, the actuator 160 moves the head 130 across tracks on the disk 110 until the head 130 is positioned at a target track, so that information can be written to, and read from, the surface of the disk.
FIG. 1B illustrates a portion of an exemplary disk 110. Information can be written in concentric tracks 170-1, 170-2, . . . 170-n on the disk 110, extending from near the inner diameter (ID) of the disk to near the outer diameter (OD) of the disk. Since a track may hold thousands of bytes of data, it may be further divided into smaller units called sectors. The tracks and sectors may be created during the low level formatting (LLF) of the disk performed during manufacturing, e.g., by self-servo writing (SSW). During self-servo writing, spirals 180-1, 180-2, . . . 180-n may be written on the disk 110 from its inner diameter to its outer diameter, and wedges 190-1, 190-2, . . . 190-n may be created. Spirals 180-n may indicate radial and circumferential data of head position relative to that of the disk 110 when the path of the spiral is known. Servo information (e.g., track numbers) may be written in servo wedges 190-n, which may be recorded on a certain number of tracks.
The head 130 seeks across the disk 110 and writes during servo spirals 180-n. SSW using spirals may use two sets of interleaved spirals: a primary set with longer interrupt service routine (ISR) time and a secondary set with shorter ISR time. Both sets will eventually overlap with wedges, but at different times. Only one set of spirals, usually the primary set, is needed to control the actuator 160. The secondary set is used to prepare for switching when the primary set is about to overlap with where the wedges are to be written and the servo information in the primary set is not accessible, so that the actuator 160 may continue to receive the servo information.
ISR time is the time to execute the time critical code to service an interrupt. When a wedge 190-n flies under the head 130, hardware of the disk drive 100 may create an interrupt and the ISR time may start. During the ISR time, a processor may decipher and translate the servo information stored in the wedge 190-n to a physical location, decide whether the head 130 is at where it is supposed to be, and whether the head 130 needs to be moved, e.g., to the left or to the right. After the ISR time, the processor may go back to what it was doing, e.g., transferring data.
The prior art methods schedule spiral ISR starts at a fixed delay from the presence of the spirals. FIG. 2A illustrates a typical scenario with prior art spiral ISR scheduling with perfect spiral placement. In FIG. 2A, a diamond is a read back signal generated when a spiral is under the head 130, and hardware of the disk drive may demodulate a spiral during a spiral demodulation window. As shown, a primary spiral set ISR starts at the falling edge of an odd (i.e., 1, 3, 5 . . . ) spiral demodulation window, and ends before half of the wedge to wedge time, which is the time between two consecutive odd (i.e., 1, 3, 5, . . . ) diamonds or two consecutive even (i.e., 2, 4, 6, . . . ) diamonds. A secondary spiral set ISR starts at a falling edge of an even (i.e., 2, 4, 6, . . . ) spiral demodulation window.
If one ISR time has not completed before the next ISR time is supposed to start, the next ISR will be skipped and the required timing/synchronization will be lost. FIG. 2B illustrates a worst case scenario with prior art spiral ISR scheduling with perfect spiral placement. As shown, to prevent synchronization from being lost, the primary ISR time cannot be longer than the spiral to spiral time or half of the wedge to wedge time, although a considerable part of the wedge to wedge time is wasted.
Spirals may be written either before or after a disk is assembled, and either method may result in spirals which are not uniformly spaced, a condition called spirals crowding. FIG. 2C illustrates a typical scenario with prior art spiral ISR scheduling with spirals crowding. As shown, to prevent synchronization from being lost, the primary ISR time has to be shorter than the shortest spiral to spiral time, with a large part of the wedge to wedge time being wasted.
Prior art approaches try to reduce the ISR time to keep it within limitations. One prior art approach for reducing SSW ISR time increases CPU clock speed. Another prior art approach uses multiple CPU cores to distribute the ISR load. Both approaches are costly.
To meet the ever-increasing storage requirement, more and more data will need to be stored on a disk. As aerial density of disks increases, tracks per inch (TPI), bits per inch (BPI) and servo sample rate also increase, resulting in shorter and shorter wedge to wedge time. In addition, spiral to spiral spacing may vary significantly from the target spacing depending on how spirals were written. These may create more constraints on the ISR time scheduling.
Therefore, it may be desirable to provide a method for scheduling SSW ISR which is more flexible and may allow higher disk data density.